Electronic circuit systems, particularly a portable system such as, for example, a wristwatch, a portable phone, and the like, widely use a crystal oscillator to generate a basic clock signal. Such a portable system is generally used under a considerably extended range of environmental conditions such as temperature in particular. Therefore, the crystal oscillator used in the portable systems is requested to operate stably in a wide temperature range.
FIG. 1 illustrates a conventional crystal-oscillator circuit. The conventional crystal-oscillator circuit includes an inverter INV1, a resistor R1, a quartz resonator XL, two capacitors C1 and C2, and a constant voltage power supplier 50. The inverter INV1 includes MOSFET (metal oxide semiconductor field effect transistor) transistors and receives a constant power supply voltage from the constant voltage power supplier 50. The MOSFET transistors of the inverter INV1 have a transfer conductance gm which needs to be constant for a stable oscillation in this crystal-oscillator circuit. However, characteristics of MOSFET transistors change in response to variations of an environmental temperature, resulting in a change of the transfer conductance gm.
When the transfer conductance gm decreases and, as a result, a loop gain of the crystal-oscillator circuit becomes below “1”, the crystal-oscillator circuit ceases to oscillate because the circuit condition is out of range for the oscillation. Meanwhile, if the transfer conductance gm increases, the circuit may perform an abnormal oscillation. Thus, there is an increasing demand to obtain an oscillation circuit which maintains a stable oscillation in a wide temperature range.